NASA officials have loosened the standards for what constitutes an acceptable risk of damage from the kind of debris that led to the disintegration of the shuttle Columbia as it was returning from space two years ago, internal documents show.

The move has set off a debate within the agency about whether the changes are a reasonable reassessment of the hazards of flight or whether they jettison long-established rules to justify getting back to space quickly.

Experts who have seen the documents say they do not suggest that the shuttle Discovery - scheduled to lift off from Cape Canaveral, Fla., on May 22 - is unsafe, but a small but forceful minority say they worry that NASA is repeating a practice that contributed to the Columbia disaster: playing down risks to continue sending humans into space.

The documents were given to The New York Times by several NASA employees, who asked not to be named, saying they feared retribution.

Documents that had been revealed earlier showed that NASA was struggling to meet safety goals set by the independent board that investigated the Columbia accident. The new documents suggest that the agency is looking for ways to justify returning to flight even if it cannot fully meet those recommendations.

The documents, by engineers and managers for the space agency, show at least three changes in the statistical methods used in assessing the risks of debris like ice and insulating foam striking the shuttle during the launching. Lesser standards must be used to support accepting the risks of flight, one presentation states, "because we cannot meet" the traditional standards.

Paul A. Czysz, emeritus professor of aerospace engineering at St. Louis University, who read the documents at The Times's request, said they did not demonstrate that the shuttle was too dangerous to return to space or that NASA was stinting on efforts to make it safer.

To achieve a profound safety improvement, he said, NASA would need to replace the shuttle fleet, which was designed in the 1970's, with an entirely new vehicle. But Professor Czysz, who spent some 30 years with McDonnell Douglas, a NASA contractor, compared the statistical shifts to moving the goal posts at a football game. "I was amazed at how they were adjusting every test to make it come out right," he said.

NASA officials say that the shuttle is safer than it has ever been because of changes made after the Columbia accident in February 2003, and they have long acknowledged that not all debris risk can be eliminated. "There is still going to be a possibility that a golden BB could get us," N. Wayne Hale Jr., the deputy director of the space shuttle program, told reporters in briefings this month.

Two years of testing since the loss of the Columbia and its crew of seven have shown that the shuttle's skin, designed primarily to resist the blistering heat of re-entry, is far more vulnerable to debris from the external fuel tank than had been thought.

The tank is filled before launching with 535,000 gallons of liquid hydrogen and liquid oxygen fuel, and insulating foam keeps the tank from icing over. From the beginning of the program, NASA rules said no foam or ice should hit the shuttle. The investigation board found that managers played down the risks over time as the craft survived thousands of blows from small pieces of debris.

After the accident, NASA officials initially expressed doubt that the 1.67-pound hunk of foam that struck the left wing could have brought it down. But tests have since proved that a 0.023-pound piece could cause catastrophic damage under the worst circumstances. NASA now says it has reduced the size of debris that will fall off of the tank to 0.01 pound or less, but admits that the only way to know is to monitor actual launching conditions.

One of the two internal documents, dated Feb. 17, 2005, was written by John Muratore, the manager of systems engineering and integration for the shuttle program, and a colleague. It describes ways to compensate for what it calls "overly conservative" assessments of the ability of the shuttle to withstand debris impacts, including these:

¶Moving from the traditional worst-case situation certification, or "worst-on-worst" approach, to "our best estimate of actual conditions."